The ITMS-FTIR network for Germany: Second year of consistent XCO2, XCH4 and XCO data for satellite and model validation on the urban, regional and national scale

Top-down estimation of greenhouse gas emissions requires the combination of reliable atmospheric concentration measurements with atmospheric inversions. The German Integrated Greenhouse Gas Monitoring System (ITMS) combines atmospheric in situ, remote sensing and satellite measurements, transport modelling, and inverse estimation techniques, aiming at an operational top-down monitoring of greenhouse gas emissions. We contribute to this effort by establishing highly consistent and accurate observations of column-average mole fractions of carbon dioxide (XCO2), methane (XCH4) and carbon monoxide (XCO) using eight COCCON (EM27/SUN) and two TCCON FTIR instruments across Germany.We operate Collaborative Carbon Column Observing Network (COCCON, EM27/SUN) and Total Column Carbon Observing Network (TCCON) spectrometers located such that the measurements cover spatial gradients on the urban, regional and national scale. We ensure excellent consistency among all stations by operating an additional EM27/SUN as travel standard, performing regular side-by-side measurements with all network instruments. As such, we calibrate all instruments to a common scale and, via TCCON, tie them to the World Meteorological Organization (WMO) scale. These measurements provide the means to validate both satellite observations and modelling results on the spatial scales relevant for future emission inversions. After the second year of operation, we present the extended dataset, as well as our solidified uncertainty analysis based on the continued side-by-side measurements throughout the years 2024 and 2025. Additionally, we present our approach to better quantify residual systematic uncertainty contributions by employing an Acetylene absorption cell during regular atmospheric measurements.